Ok, I have another scenario where one might want to merge several power supplies. But in this case we need not worry about a load that is higher than the power supplies are rated for.
It's about a UPSes. The ones that you buy in a consumer store are not designed to sustain a voltage for very long durations whenever there is an outage. So it is tempting to swap the small lead-acid batteries with car batteries or marine/deep-cycle/AGM batteries that are considerably larger. For
simplicity I assume that this "cheap" UPS can handle longer durations than it is rated for and sustain a given load for extended periods of time during outage, when using the larger batteries.
A PSU also have a battery charger, depending on how it is designed, this charger may not like to handle bigger batteries. A common concern is that a smart battery charger may have circuitry that monitors the health of the batteries. A larger battery takes longer time to recharge so a battery health monitor may assume that this longer duration is due to a bad battery.
A solution to this I figure is to add a smart charger circuit onto the batteries inside the UPS. I myself have an APC SmartUPS 5000 and a Mikrobak 2k1 (nice toys, I know
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/wink.gif)
). The APC has four banks of lead acid batteries where each bank consists of four 1280 batteries connected in series, yielding 48V per bank. That gives about 1.5k Watt hours minus inefficiencies in the inverter circuit. The Mikrobak has one bank of four NP38-12, NP40-12 or 1240D batteries. That gives about 1.9 kWhrs minus inverter losses. Also, the APC is rated to sustain a load of 5000VA and the Mikrobak to sustain a load of 2100VA.
I figure that I could replace these banks with one bank consisting of four 75Ah AGM batteries. That would about more than double that capacity of those UPSes, but their charging circuitry might not be so fond of this replacement. So a solution could be to add a 48V smart-charger on top of this bank while putting the smart-charger behind a rectifier circuit to prevent current from flowing the wrong way.
How does that sound as a solution? Perhaps if a self-diagnostics test is made on a load of bad batteries, the smart-charger may not be so happy about it. I think at least the SmartUPS does that routinely once per week.